1. Field of the Invention
the invention relates to a circuit arrangement for processing a color signal reproduced from a record carrier, preferably a video tape, and modulated on a carrier wave of a first carrier frequency (color-under signal), in which circuit arrangement the color signal modulated on the first carrier frequency being applied to a first mixer which converts the color signal to a second carrier frequency by mixing it with a first mixing signal and whose output signal is applied to a comb filter circuit, the output signal of the comb filter circuit being applied to a second mixer which converts the comb-filtered signal to a third carrier frequency by mixing it with a second mixing signal.
2. Description of the Related Art
European Patent EP-A-0 752 791, corresponding to U.S. Pat. No. 5,802,238, discloses a circuit arrangement by which a color signal, reproduced from a record carrier, particularly a video tape, and modulated on a carrier wave of a first carrier frequency, is comb-filtered and the color carrier frequency is converted to a frequency in accordance with a reproduction standard of the color signal. For. example, in the case of the VHS standard, the color signal is modulated on a color carrier having a frequency of 625 kHz when it is recorded on the tape. In the case of, for example, the NTSC standard, this signal should be re-converted to the color carrier frequency of 3.58 MHz. In addition to this conversion of the color carrier frequency, this known circuit arrangement performs a comb filtering. A first mixer has been provided, which converts the color carrier frequency to an intermediate value at which comb filtering is effected. After comb filtering, the conversion to the color carrier frequency in accordance with the reproduction standard is effected by means of a second mixing process. Comb filtering is effected to suppress the effect of crosstalk from adjacent tracks on the record carrier. By means of the two mixers, it is achieved that comb filtering is effected within the frequency-controlled and phase-controlled range of the circuit arrangement and, moreover, that the circuit complexity remains low because the color signal is still available with a carrier frequency below the carrier frequency required for reproduction.
In this circuit arrangement, as in other circuit arrangements in which comb filtering is effected at other frequencies, the problem arises that the period of the carrier wave on which the color signal is modulated, is not an integral multiple of the duration of one picture line of the color signal. As a result of this, the problem arises that comb filtering is effected with color carrier waves which are shifted in time with respect to one another, i.e., the color carrier waves of the picture lines to be filtered are phase-shifted with respect to each other during comb filtering. This prevents an optimum filtering. The problem that the period of the color carrier is not an integral multiple of the line period, occurs for all three carrier frequencies.